TripAdvisor.com, for its usefulness in planning a vacation. I used it to plan our honeymoon and it couldn't have been more spot on in terms of hotel reviews and area summaries. In fact, both hotels that I booked using TripAdvisor were amazing experiences. La Villa Marbella was a wonderful little B&B, located in the old town but within a ten-minute walk of the beach. The service there, along with the rooms, was first-class. Casa Camper was a revelation, located in the heart of Barcelona's artsy Raval district. Truly my kind of hotel.
The third hotel, which I got a good deal on, but is part of a 5-star chain, was just OK.